The present invention relates to improvements to radar or sonar echo detection systems making it possible to provide an image of the surrounding space in real time. The apparatus according to the invention is more particularly applicable to high definition sonar equipment by the formation of channels, such as acoustic cameras and target classification systems in submarine technology, imaging taking place in frontal vision. The invention also applies to medical acoustic imaging and to non-destructive tests.
Radar or sonar systems are generally constituted by at least one transmitter, whose transmitted signals "illuminate" the angular field to be observed and a receiving transducer array, whose signals received are used for forming a system of angular channels covering this field. To each channel formed corresponds a distribution of the energy received in the space, whose angular width of the major lobe at the half power defines the angular resolution.
It is known to bring about imaging by means of transmitting and receiving arrays formed by lines of transducers in orthogonal directions. The signals received about a frequency f by the transducers of the receiving array are delayed or phase-displaced relative to one another to form channels in directions forming a constant angle with the direction of said receiving array.
The signals transmitted about frequency f by the transducers of the transmitting array are delayed or phase-displaced relative to one another to form channels in directions forming a constant angle with the direction of the transmitting transducers. The formation of the transmission channels takes place sequentially in time.
Thus, fine transmission and reception channels are successively obtained with angular widths of the 3 dB attenuation diagrams=.lambda./L, in which .lambda. is the wavelength corresponding to the frequency f in the propagation medium and L the length, which is assumed to be equal, of the transmitting and receiving arrays.
This prior art apparatus for forming fine angular channels has the disadvantage of a low information rate, because transmission must take place on all the transmitters the same number of times that there are transmission channels.
It is also known to carry out acoustic imaging with a single transmitter which gives the desired angular field. The receiving array in this arrangement is formed by several lines of transducers. Processing of all the signals received by the transducer makes it possible to form fine angular channels in two directions. If a large number of channels is to be formed, which is generally the case when it is desired to obtain a fine angular resolution, there is a large number of receivers and the volume of the electric circuits is also considerable.
It is known to form channels by using acoustic lenses. However with this process the angular field remains limited to a few degrees and requires a "retina" in the focusing plane.